There are various methods for measuring the distance using an image capture device. For example, according to one method, the distance is estimated based on stereoscopic parallax between two images which have been captured by two image capture devices (or image sensors). According to another method, the distance is be estimated by projecting a special dot pattern into a space and analyzing its image. The distance is also estimated by a so-called TOF (Time of Flight) method in which the object is irradiated with optical pulses modulated at high frequencies and the phase difference between the forward and backward optical pulses is measured.
Patent Document No. 1 discloses another exemplary distance measuring technique. The device disclosed in Patent Document No. 1 makes a single image sensor capture a plurality of images by projecting light time-sequentially from multiple light sources onto a target (or object) which is either standing still or moving. The distance to the object is measured based on the luminance ratio of these images.
However, such a method that requires capturing a plurality of frames as disclosed in Patent Document No. 1 is far from being perfectly suited for a motion sensor which shoots a moving target. Nevertheless, this problem can be overcome by using a sensor device such as the one disclosed in Non-Patent Document No. 1. Specifically, Non-Patent Document No. 1 discloses a sensor device which can capture a plurality of images without making a read transfer even though the number of frames captured falls within its limited number and almost without causing a time lag.
If the distance measuring technique as disclosed in Patent Document No. 1 is applied to a motion sensor device, then the measurement involves turning multiple light sources ON, thus dissipating a lot of power, which is a problem.
Thus, to overcome such a problem, people proposed techniques for measuring the distance with the power dissipation cut down. Here are some examples:
Patent Document No. 2 discloses a device for getting distance information by the TOF method. This device cuts down power dissipation by reducing the optical output power of its light source unit according to the distance to the object of measurement.
Patent Document No. 3 discloses a technique which uses a pyroelectric sensor in order to cut down standby power unless the object of detection is present within the measuring range.